Enterococcal infection is a leading health concern. They rank among most common causes of hospital acquired infection, and the enterococci that cause these infections are often resistant to multiple antibiotics - and with increasing frequency, to all antibiotics. After examining available data, we recently proposed that most nosocomial enterococcal infections are the result of a 2 step process: 1) virulent hospital strains of antibiotic resistant enterococci colonize the gastrointestinal tract of the patient, where their numbers are greatly amplified; followed by 2) infection of extra intestinal sites, such as the bloodstream, urinary tract, or surgical wound. Studies of GI tract colonization and the transition to infection are intractable because of their complexity. There are an estimated 500 taxa represented among bacterial species in the colon. Moreover, species representation in this consortium varies by age and diet, and is further confounded by the health status of the host and antimicrobial therapy. We recently found that Drosophila are naturally colonized by enterococci, that these bacteria occur in the posterior intestine, and that they are represented in proportions similar to that found in the human colon. Development of a simple, tractable model to study the complex interactions between enterococci, other organisms of the GI tract consortium, and the host, would represent a substantial advance in the effort to develop rational approaches for limiting the occurrence and spread of multiple antibiotic resistant strains of enterococci among hospitalized patients. The aims of this R-21 Exploratory Proposal, which was submitted in response to a call for proposals from the National Center for Research Resources, are to verify preliminary observations on the natural colonization of Drosophila by enterococci, to determine whether replacement of indigenous enterococci with clinical isolates that vary in virulence potential (as would be needed to use this model to study enterococcal virulence and pathogenesis) impacts the physiology of the fly or colonization pattern, and to determine the extent to which antimicrobial peptides produced by Drosophila regulate colonization pattern of the GI tract.